A thing to note about how this can play out in different scenarios:
If one of the units slated to join the cluster relation are delayed in finishing their other relations for whatever reason, the Keystone units waiting for it to join will continuously run their cluster-relation-changed hook until it is able to complete joining.
A thing to note about how this can play out in different scenarios: relation- changed hook until it is able to complete joining.
If one of the units slated to join the cluster relation are delayed in finishing their other relations for whatever reason, the Keystone units waiting for it to join will continuously run their cluster-